1. Field of the Invention
The present invention relates to a heat treatment apparatus for irradiating a substrate including a semiconductor wafer, a glass substrate for a liquid crystal display device and the like with light to perform a heat treatment on the substrate, and more particularly to a heat treatment apparatus for emitting a flash of light to momentarily heat the substrate.
2. Description of the Background Art
Conventionally, a lamp annealer employing a halogen lamp has been typically used in the step of activating ions in a substrate after ion implantation. Such a lamp annealer carries out the activation of ions in the substrate by heating (or annealing) the substrate to a temperature of, e.g., about 1000° C. to 1100° C. Such a heat treatment apparatus utilizes the energy of light emitted from the halogen lamp to raise the temperature of the substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a substrate by the use of the above-mentioned lamp annealer which raises the temperature of the substrate at a rate of about hundreds of degrees per second produces a phenomenon in which the ions of boron, phosphorus and the like implanted in the substrate are diffused deeply by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, giving rise to apprehension about a hindrance to good device formation.
To solve the problem, there has been proposed a technique for irradiating a surface of a substrate with a flash of light by using a xenon flash lamp or the like to raise the temperature of only the surface of the substrate, which is implanted with ions, in an extremely short time (several milliseconds or less). The xenon flash lamp has a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from the conventional halogen lamp, and almost coincides with a fundamental absorption band of a silicon substrate. It is therefore possible to quickly raise the temperature of the substrate, with a small amount of light transmitted through the substrate, when the substrate is irradiated with a flash of light emitted from the xenon flash lamp. Also, it has turned out that the flash light irradiation in an extremely short time of several milliseconds or less can selectively raise the temperature of only near the surface of the substrate. Therefore, the temperature rise in an extremely short time by using the xenon flash lamp allows the execution of only the ion activation without deep diffusion of the ions. Such a constitution example of heat treatment apparatus employing a xenon flash lamp is shown in, e.g., Japanese Patent Application Laid Open Gazette No. 2004-140318.
Before flash heating using the xenon flash lamp, preheating is performed on the substrate by using a hot plate, a halogen lamp or the like. Though the hot plate has the advantage of being safe and raising the temperature of the substrate uniformly to some degree, it limits the scope of application since it has a limitation of possible raised temperature. On the other hand, using the halogen lamp allows a rise in temperature of 600 degrees or more. If it is possible to preliminarily raise the temperature of the substrate to a high-temperature range of 600 degrees or more before the flash heating, the range of temperature rise required for the flash heating can be thereby reduced. In other words, the energy to be given to the substrate by the xenon flash lamp can be reduced. This reduces the thermal stress imposed on the substrate in the flash heating, and it is thereby possible to prevent the substrate from being broken by the flash heating.
In the apparatus employing a halogen lamp to perform the preheating, however, a tube wall of a xenon flash lamp absorbs rays of light emitted from the halogen lamp in the preheating and this disadvantageously raises the temperature of the tube wall. Since the life of the xenon flash lamp depends on the temperature of its tube wall, the rise in temperature of the tube wall remarkably deteriorates the flash lamp.